A control system for motor-operated power machines is known which ensures operator safety by instantaneously interrupting power to the motor driving the rotary tool, such as a blade, saw or the like, when an electrically conducting metal glove worn by the operator or his hand makes contact with the tool or other conducting part of the machine. The conducting metal glove is covered with an insulating sheath, such as a glove plastic sheet glove, to prevent electrical connection being made via for example a bulk of meat handled by the operator.
The operator's glove, or a conducting arm band worn by him, is electrically connected to a safety switch which instantaneously disconnects the power when the operator's glove or hand closes a circuit by touching a part of the machine.
The control system comprises means for reversing the polarity of the electric motor, and further reduces the possibility of damage to the operator by, if appropriate, rotating the motor in the reverse direction through a fraction of a revolution. A manual switch has to be operated to reset the configuration of the safety circuit to its state prior to intervention. Each metal glove is connected to the safety switch by an electrode fixed to the operator's wrist in proximity to the glove, and the two electrodes have to be worn by the operator to make it possible to connect current to the motor for the rotating element.
Such a system is fully described in various embodiments in U.S. Pat. No. 4,965,909 in the name of the present applicants.
The aforesaid system requires the operator to wear two electorally conducting gloves, which are connected via electric wires provided with suitable plugs to corresponding sockets in the body of the power machine, such as a skinning machine.
The plugs and sockets are of coaxial type in the sense that they comprise a pin plus a central receiving seat for connection to one wire, and a coaxial skirt external to the pin plus a receiving cavity for connection to the other wire.
Said two wires are connected to a safety circuit such as that shown in FIG. 6 of said U.S. Pat. No. 4,965,909 which is contained in the base of the machine and receives current from the general machine circuit shown in FIG. 5 of said European patent application.
Furthermore in the system illustrated in said U.S. Patent only one glove is directly connected to the control circuit, the other glove being connected electrically to the first via the operator's body; a single control circuit thus responds to the pulses emitted by both gloves. Although having proved effective, the system illustrated in said U.S. Patent has a series of drawbacks as listed below.
First, the electronic circuit which receives the signals from the electrically conducting gloves is powered, via a transformer, by the general machine circuit connected to the mains, with the result that abnormalities or malfunctions could arise, producing electrical discharges across the gloves, with imaginable consequences for the operator.
The possibility, although remote, of return currents dangerous to the operator, makes it also very difficult to obtain certification for the system in those countries in which safety codes are most severe.
In addition, the plug of the coaxial cable which connects the gloves to the machine structure is highly sensitive to moisture, and any occurring condensation can short-circuit the two poles of the cable, to enable the machine to run even when the operator is not wearing the gloves.
This phenomenon is of considerable annoyance because skinning machines, to which the invention is mostly applicable, operate in moist environments in which considerable use of water is made. Lastly, as the two metal gloves are connected together by the operator's body but only one of the gloves is connected to the safety circuit, they have proved to have different sensitivities, the glove directly connected to the circuit being much more sensitive than the other glove.
The following improvements are proposed according to the present invention to overcome the aforesaid drawbacks.
First, the electronic safety circuit which generates the pulses and feeds them to the machine electric circuit to interrupt power to the motor, is powered by an independent 12 V. d.c. battery and is no longer located in the base of the machine but on the operator.
Secondly, said electronic circuit no longer serves both gloves, and instead an independent circuit is provided for each glove. Lastly, the signals or pulses of the electronic safety circuit are transmitted to the machine electric circuit by an optical fibre.